Many data storage media such as optical media or magnetic media have been developed to store data. With the increasing development of digitalized generation, the data storage density for the conventional data storage media is unsatisfactory. Nowadays, for dealing with such a problem, a holographic storage technology is developed by using holographic storage media to store data.
Referring to FIG. 1, the basic concept for implementing a holographic storage technology is shown. An object beam 11 is imprinted with pixel data of a two dimensional array through the encoding operation of a spatial light modulator (SLM) 10. Then, the resulting object beam 11 passes through a lens 16 and interferes with a reference beam 12 to form an interference pattern which is, afterwards, recorded into a holographic storage medium 13. The pixel data recorded in the holographic storage medium 13 can be read out by extracting the object beam with pixel data from the interference pattern.
In the data readout process, the interference pattern previously recorded in the holographic storage medium 13 is subjected to diffraction with the reference beam 12 to generate a reconstructed object beam 15. The reconstructed object beam 15 is directed through a lens 17 and focused on an optical detector 14 such as a charge coupled device. The reconstructed object beam 15 is decoded by the optical detector 14 via photoelectric conversion thereby realizing the pixel data.
In order to further enhance data storage capacity, a volume holographic storage technology was developed. The volume holographic storage technology utilizes multiplexing means to store a large number of holograms in the same spatial region of a holographic storage medium. Angle multiplexing is one of the volume holographic storage technologies for storing a large number of holograms in a single storage medium.
Referring to FIG. 2, an angle multiplexing holographic storage device is schematically shown. The angle multiplexing holographic storage device 200 comprises a laser beam 210, a beam splitter 215, a turning mirror 245, a pattern encoder 255, a lens 280, a holographic storage medium 250, a rotatable address selection mirror 286, an elliptical mirror 288, another lens 284 and an optical detector 281. By the beam splitter 215, a coherent light from the laser source 210 is split into two beams to serve as a reference beam 220 and an object beam 225, respectively. The reference beam 220 is reflected by the rotatable address selection mirror 286 to an elliptical mirror 288. The elliptical mirror 288 has two focal points. The rotatable address selection mirror 286 and the holographic storage medium 250 are located at the first and second focal points of the elliptical mirror 288, respectively. The reference beam 220 reflected from the elliptical mirror 288 is focused on the holographic storage medium 250. The object beam 225 is processed through the reflection of the turning mirror 245 and the encoding operation of the pattern encoder 255, e.g. a spatial light modulator (SLM) to imprint thereon pixel data, and then focused on a particular site on the holographic storage medium 250 via the lens 280. The data-imprinted object beam 225 interferes with the reference beam 220 to form an interference pattern to be recorded in the holographic storage medium 250. By rotating the address selection mirror 286, the incident angle of the reference beam 220 in respect with the holographic storage medium 250 is adjustable. The interference patterns resulting from a certain object beam 225 and reference beams 220 with various incident angles are recorded at the same site on the holographic storage medium 250. In such manner, data storage capacity of the holographic storage medium 250 is largely improved.
For reading pixel data from the holographic storage medium 250, the address selection mirror 286 is rotated differentially to render a reference beam 220 with a desired incident angle. Through the diffraction of one of the interference patterns recorded in the holographic storage medium 250 with the rendered reference beam 220, a reconstructed object beam 282 can be obtained. Then, the reconstructed object beam 282 is directed through the lens 284 and focused on the optical detector 281, which can be a charge coupled device, to be decoded into the pixel data via photoelectric conversion.
Since the angle multiplexing holographic storage device stores holographic data by adjusting the incident angle of the reference beam 220, the rotation of the address selection mirror 286 has to be precisely controlled. It is known to those skilled in the art, however, rotation control is far more difficult than linear control in precision. In other words, the incident angle of the reference beam 220 in respect with the holographic storage medium 250 is hard to be finely adjusted. Therefore, the yield and reliability of the product would be adversely affected.